1. Field of the Invention
The present invention pertains to a method for creating printing data applied to a printer that is capable of generating ink droplets of different sizes, and more particularly to a method in which a halftoning threshold table is created and the result of the halftoning process is encoded in the form of a series of binary bits, wherein these binary bits can directly drive the printer to facilitate the efficiency of the printer.
2. Description of Related Art
So-called “Drop-on-Demand Ink Jet Printing” is achieved by ejecting drops of ink at the desired positions on the proper material to create a printed image when an ink jet cartridge is moving back and forth on the media. In order to create the images having “photographic quality”, there should be plenty of density levels for each given pixel. That means the color density levels printed on a single pixel for a color such as cyan, magenta or yellow (C,M,Y) should be programmable. Through the variety combinations of different colors (C,M,Y) in company with the variation of the density levels, lots of colors can be derived to create printed images achieving photographic quality.
The different density levels can be easily accomplished by a multi-drop printing technique. It is understandable that by repeatedly applying multiple drops of ink onto a pixel, color density can be gradually increased whereby any desired density level is able to be created. In order to repeatedly apply multi drops of ink at the same position, the ink jet printhead must move back and forth above the material to be printed. However, such a multi-pass operation for generating high quality images usually requires a lot of time to accumulate enough drops of ink for a single pixel.
In order to acquire photo image quality and to create an appearance of digitized continuous tone images, the quantity of drops of ink deposited on a pixel should be increased. For early printing techniques, the size of each drop is approximately 100 pico-liter. For example, if the printing resolution is 300 dots per inch (dpi), a pixel would almost be completely occupied by only a single drop of ink so that only a few colors can be generated. Hence, the size of each drop must be minimized as far as possible. To overcome the problem, Hewlett-Packard™ developed a photo resolution enhancement technology wherein the present color inkjet printhead (three colors, CMY) has the ability to output a 10 pico-liter drop. Therefore, there are eight drops of ink at most that can be used to construct a single pixel. By applying a different number of drops, for example 0, 1, 3 or 8 drops on a single pixel, four different color density levels from the lightest (white color) to the darkest can be gradually accomplished. If the three primary colors (CMY) are further mixed, more than 250 different colors can be obtained.
For an inkjet printer, image data to be printed is firstly processed by halftoning technique to create the digitized continuous tone images. Since the early printing technique is limited by the large size of the ink drop, the halftoning process only determines that a dot is either deposited or it is not. In this situation, if each pixel is represented by 8-bits digitized data to stand for 0 to 255 continuous density levels, the middle level (i.e. 127) would be adopted as a halftoning threshold value. For any pixel having a density level that exceeds the threshold value, it would be deposited with a drop of ink. Otherwise, for a density level lower than the threshold value, there is no ink deposited on that pixel. Through the halftoning process, the original 8-bits data for representing density level can be simplified to one bit (0/1) that stands for whether the ink should be deposited or not.
As mentioned above, each pixel may be presented by multiple drops of ink to express different density levels and to achieve the photo image quality. Therefore, whether the ink drop is ejected is not the only concern, the information of the depth of color is another point that should be noticed. If the size of ink drops is uniform, the color depth is only determined by the number of the drops applied on the pixel. Obviously, one bit information is unable to represent the multi density levels for a single pixel.
If two bits are adopted to represent four density levels 0 to 3, the digitized continuous tone value 0–255 would be divided into four stages, wherein three threshold values are set at 63, 127 and 191. Therefore, any tone value within any one of the four stages (0–63), (64–127), (128–191) and (192–255) would correspond to a particular density level (0, 1, 2 or 3). The four density levels are sequentially represented in binary form by two bits, i.e. 00, 01, 10 and 11. Such a halftoning process utilizes more than one bit, therefore it is also referred to as “multi-level halftoning process” or “multi-bit halftoning process”.
After the halftoning process, each pixel has a corresponding color density level. The next step is to determine how many drops of ink are enough for the density level. The number of the drops can be decided by the following sequences.
Firstly, an allowable maximum quantity of the drops of ink that can be deposited on a single pixel should be determined. If the ink drops applied on paper exceed the allowable number, the variation of the color density is unable to be recognized, or the drops of ink would exceed the area covered by a single pixel. In the above example, the allowable maximum quantity of the drops is 8. Secondly, during the process that the ink is deposited at the same position on paper one drop by one drop until the allowable maximum quantity is reached, the color density can be observed and measured. Since there are four levels for presenting different color densities in this example, 0%, 33%, 66% and 100% color density respectively correspond to zero drops, one drop, three drops and eight drops {0, 1, 3, 8}. As shown in FIG. 4, Y axis stands for the color density and X axis represents the quantity of the drops of ink. According to FIG. 4, any tone value within one of the four stages (0–63), (64–127), (128–191) and (192–255) would respectively correspond to zero drops, one drop, three drops and eight drops {0, 1, 3, 8}.
In the foregoing description, all the drops of ink ejected from the ink jet are identical in size. However, if an inkjet printer has the ability to provide different sizes of ink drops, a printing job would be quickly finished by a proper combination of these ink drops with different sizes, wherein the required total number of the ink drops is the same, but the number of the drops is much less than the previous way.
As disclosed in U.S. Pat. No. 5,917,510, a drop-on-demand ink jet printhead controlled by the piezoelectrical method is able to output ink dots with different sizes according to the result of the halftoning process. The halftoning process adopted in that patent is the so-called “error diffusion”. The printhead for the printer is capable of being controlled to eject ink drops of different sizes. Accordingly, the design of such an inkjet printhead is more complex than ordinary types. Moreover, the fabrication cost for the printhead is high. Further, the density level obtained from the halftoning process must be further analyzed to determine how many drops of ink should be ejected from which printhead. Although the printer is able to provide the ink drops of different sizes, the printing speed is still hampered by the corresponding halftoning process, printhead selection and the ink drops determination.